1. Field of the Invention
The present invention relates to an optical glass having optical constants in the form of a refractive index nd of 1.70 or higher and an Abbé number nud of 50 or higher, a preform for precision press molding comprised of this glass, an optical element comprised of this glass, and methods for manufacturing the preform and the optical element.
2. Discussion of the Background
With the appearance of digital cameras and portable telephones equipped with cameras, a high degree of integration and functional development has taken place rapidly in the apparatuses in which image pickup optical systems are mounted. At the same time, the demand for high-precision, lightweight, compact optical systems has intensified.
To meet this demand, optical designs employing aspherical lenses have become the mainstream in recent years. Thus, precision press molding techniques (also known as mold pressing techniques) of forming optically functional surfaces directly by press molding without a grinding or polishing step have attracted attention as methods of stably providing large quantities of inexpensive aspherical lenses employing highly functional glass, and the demand for optical glasses having low temperature softening properties that are suited to precision press molding has increased each year. Such optical glasses include high-refractive-index, low-dispersion glasses. An example of such a glass is described in Japanese Unexamined Patent Publication (KOKAI) No. 2002-249337, or English language family member U.S. No. 2003125186 A1, which are expressly incorporated herein by reference in their entirety).
To make full use of the advantages of such precision press molding techniques, it is desirable to directly manufacture a glass material known as a “preform for press molding” from a glass melt. This method, known as the preform hot forming method, comprises causing a glass melt to flow out, successively separating gobs of glass melt in quantities corresponding to single preforms, and cooling the glass melt gobs obtained to form preforms with smooth surfaces. Accordingly, this method affords advantageous characteristics in the form of a better glass use rate than methods in which large glass blocks are formed from glass melt and the blocks are cut, ground, and polished; in that no glass scraps are generated during processing; and in that less time and cost are incurred in processing.
In the hot forming method, a gob of glass melt in a quantity corresponding to a single preform must be accurately separated, and the preform must be formed without causing defects such as striae and devitrification. Accordingly, hot forming requires a glass having good glass stability in the high temperature range.
When raising the refractive index nd while maintaining an Abbé number nud at or above a certain value, the tendency of the glass to crystallize intensifies, and, as a result, vitrification becomes difficult. Further, crystals in the glass tend to precipitate out during the heating and softening steps in precision press molding. Imparting even softening properties at a lower temperature to the glass employed in precision press molding tends to diminish glass stability. Accordingly, it has been difficult to raise the refractive index nd to 1.70 or higher while maintaining an Abbé number nud of 50 or higher, desirably 52 or higher. It has also been difficult to achieve good resistance to devitrification during precision press molding and a level of glass stability permitting hot forming of preforms while imparting low temperature softening properties suited to precision press molding.
The present invention, devised to solve the above-stated problems, has for its object to provide an optical glass that has a refractive index nd of 1.70 or higher, an Abbé number nud of 50 or higher, and low temperature softening properties, and that exhibits good glass stability; a preform for precision press molding comprised of this glass; a method for manufacturing this preform; an optical element comprised of this glass; and a method for manufacturing this optical element.
The present inventors conducted extensive research into the thermal characteristics of optical glasses in determining optical glass compositions. They discovered that the low temperature softening properties and glass stability of an optical glass could be evaluated by measurement with a differential scanning calorimeter (DSC). A differential scanning calorimeter scans the temperature of a glass sample over a broad temperature range, measuring the heat generation and heat absorption of the sample at a variety of temperatures. Hereinafter, a temperature 120° C. higher than the glass transition temperature Tg will be denoted as “Tg+120° C.” and a temperature 100° C. lower than the liquidus temperature LT will be denoted as “LT−100° C.”
During precision press molding, the glass is generally maintained within a temperature range at or above the glass transition temperature Tg but not exceeding (Tg+120° C.). Glass in which crystals precipitate within this temperature range generate exothermic heat during crystallization. That is, crystals will precipitate during precision press molding in glass having an exothermic peak within this temperature range. Accordingly, the present inventors set as their first object the obtaining of a glass exhibiting no exothermic peak within a temperature range of greater than or equal to the glass transition temperature but not exceeding (Tg+120° C.) (in which the level of exotherm generated by the sample does not reach a maximum during scanning of this temperature range) to obtain an optical glass having high glass stability.
The present inventors set as their second object the obtaining of a glass in which just one endothermic peak was present within the temperature range of greater than or equal to (LT−100° C.) but not exceeding the liquidus temperature LT to obtain an optical glass having good low temperature softening properties. The glass is molten over this temperature range. In differential scanning calorimetric measurement, endothermic peaks generated within this high temperature range originate from the absorption of heat occurring simultaneously with the melting of crystals that have precipitated within the glass. The present inventors examined the relation between endothermic peaks produced in the high temperature range and glass stability in the course of molding a glass melt. They observed that, when an index A of the refractive index was defined asA=nd−2.25−0.01×nud, as the endothermic peak temperature differential of the respective glasses decreased, the liquidus temperature LT tended to decrease in glasses with comparable levels of index A, equal glass transition temperatures Tg, and multiple endothermic peaks within the temperature range greater than or equal to (LT−100° C.) but not exceeding the liquidus temperature LT. This tendency varied with the content of high refractive index components, and the ratio of the contents of these components, within the glass. Thus, optimizing the composition of high refractive index components to achieve a single endothermic peak was found to yield a glass with less tendency to crystallize during the outflow of glass melt at a given outflow temperature.
Further, when comparing the thermal characteristics of glasses with indices A of identical level and identical glass transition temperatures Tg, it was discovered that the property of exhibiting no exothermic peak within the temperature range greater than or equal to the glass transition temperature but not exceeding (Tg+120° C.), desirably the property of a low crystallization exothermic peak intensity (referred to as “low temperature stability”), and the property of the presence of just one endothermic peak within the temperature range of (LT−100° C.) or greater but not exceeding the liquidus temperature LT (referred to as “high temperature stability”) were interrelated; it was discovered that increasing either the high temperature stability or the low temperature stability made it possible to increase the other.
Accordingly, the present inventors conducted further extensive research with the aim of obtaining a high-refractive-index, low-dispersion glass having the above-stated thermal characteristics. They discovered that in glasses in which B2O3 was incorporated as a network-forming component, rare earth components and the like were incorporated to impart a high refractive index and low dispersion characteristics, and Li2O was incorporated to lower the glass transition temperature without compromising the high refractive index and low dispersion characteristics, the stability of the glass depends on which rare earth component(s) is introduced. That is, when prescribed quantities of La2O3, Gd2O3, and Y2O3 were incorporated, a glass with a higher refractive index, higher dispersion, and greater stability was obtained.
The present inventors conducted further examination based on this information, and were thus able to devise the present invention.